Histones are the protein portion of a protein-DNA complex termed the nucleosome. The acetylation of the .SIGMA.-amino group of specific lysines present in the amino termini of histones has been correlated with both increased and decreased gene activity.
Nucleosomes structurally organize chromosomal DNA to form chromatin. The degree of interaction between histones and DNA varies between regions undergoing transcription and regions not being transcribed. The histones in chromatin regions containing active promoters are often post-translationally modified with acetyl groups covalently attached to specific lysine residues.
Hyperacetylated histones are thought to adopt a chromatin structure that allows other proteins to bind promoter DNA and activate transcription. Inactive promoters are associated with hypoacetylated histones, and removal of the acetyl groups from histones in normally active chromatin will repress transcription in that region.
Histone deacetylase (HD), responsible for removing acetyl modifications, may be localized to promoters targeted for repression by other proteins that associate with HD and specifically bind regulatory elements in promoter DNA.
Crop losses from pathogen infections are substantial and consume considerable quantities of productive plant biomass. It is generally believed that plant pathogens must find a way to suppress elicitation, the mechanism by which an elicitor, a pathogen-derived compound, induces disease gene expression upon recognition by the host.
One necrotrophic pathogen, the filamentous fungus Cochliobolus carbonum race 1, synthesizes a cyclic tetrapeptide, HC-toxin. HC-toxin is absolutely required for pathogenicity and is a specific inhibitor of HD activity. Resistant maize genotypes produce an HC-toxin reductase encoded by the nuclear Hm locus, which abolishes toxin activity by reducing the ketone group. These plants develop small expanding lesions in response to inoculation with Tox2.sup.+ isolates of C. carbonum, similar to the lesions formed by Tox2.sup.- isolates regardless of the host genotype. HC-toxin acts in a cytostatic manner. It is not toxic to plant cells and does not determine pathogenicity by simply killing host cells prior to colonization.
Acetylation thus plays a key role in gene activation and in some instances invasion by pathogens. Mechanisms are therefore needed to control acetylation that may control gene activity and potentially play a role in disease resistance.